I suspect they were an intermediate "fad" and a cheaper way to make TVs with screens bigger than they could practically do with other available technologies at the time. This makes even more sense if you consider that the tube technology was already there for CRT-based projectors, and simply needed to be adapted to a TV-set.
Then came LCDs, which are all the rage now, and can essentially be manufactured in any size and scale, since they don't have the engineering problems (weight, size, sweep speeds necessary) you get with larger CRTs.
Biggest PITA with projection sets is changing coolant/tubes and the hassle to re-align everything afterwards... that, and replacement tubes aren't exactly cheap.
Of course, now we see DLP projection TVs, which have their own set of problems... but at least the one bulb is cheaper than 3 new projection CRTs.
Edit: If I recall, my Uni offered a projection-tv-based smartboard system up for grabs on FreeCycle... it was still there three months later. I suspect a part of the problem was just that it's size made it rather difficult to transport... but still, FREE, and nobody wanted it. (1-day U-haul rent if you don't have a truck - still a good deal!).